1. Field of the Invention
This invention relates in general to the field of steam generators for commercial nuclear power plants and in particular to apparatus for preventing vibration of the tubes of steam generators and more particularly to apparatus for eliminating clearance space between the tubes of a steam generator and the antivibration bars disposed between the columns of the tubes and thereby eliminating the vibration of said tubes during operation of the steam generator.
2. Description of the Prior Art
Nuclear power plants have been safely producing electricity for many years. The principal of operation of such commercial nuclear power plants is well known. A nuclear core containing fissionable fuel is caused to achieve criticality and thereby produces heat. The heat is removed by a reactor coolant, which in the field of pressurized water reactors, comprises water. The water reactor coolant also serves as a nuclear moderator which thermalizes fast neutrons in order to enhance the probability of the neutrons producing additional nuclear fissions and thereby sustaining the nuclear reaction. Since the chain reaction is dependent upon the presence of the nuclear moderator, the absence of the same stops the chain reaction and shuts down the reactor. This is only one of the inherent safety features of a water-cooled nuclear reactor which contributes to the overall high safety factor of such reactors.
The heat produced by the nuclear core is transferred to the reactor coolant as it passes through the nuclear reactor core. The reactor coolant subsequently transfers the heat it has received to another medium, which also comprises water and which is transformed into steam. The steam is then used to generate electricity by conventional steam turbine-electrical generator apparatus.
The reactor coolant transfers its heat to the secondary medium in steam generators specifically designed for the nuclear power field. The design of such nuclear steam generator is well known in the art. In general, the steam generator design comprises a plurality of small diameter tubes which are housed within a pressure-bearing container in such a manner as to allow and promote the transfer of heat to produce steam.
In particular, the design of some nuclear steam generators includes an outer shell comprising an elongated, circular cylinder having rounded ends attached thereto. A large number of U-shaped tubes oriented along the longitudinal axis of the cylinder, are disposed in the lower cylindrically-shaped portion of the steam generator. The lower portion has a lower or bottom end thereof associated with a closure head typically of a hemispherical configuration. The closure head is divided by a partition into a first half typically known as the hot leg, and a second half typically known as the cold leg. The high temperature reactor coolant from the nuclear reactor is input into the steam generator through a primary coolant inlet nozzle into the hot leg. The reactor coolant then flows from the hot leg into the exposed openings of the plurality of U-shaped tubes, through the tubes and then through the cold leg portion of the closure head. Finally, the reactor coolant exits from the steam generator through a primary coolant outlet nozzle.
The portion of the steam generator primarily including the bundle of U-shaped tubes and the closure head is typically referred to as the evaporator section. The steam generator further includes a steam drum section which is located at the upper end of the cylindrical shell of the steam generator. Moisture separators are located within the steam drum section. Feedwater (which comprises recirculating water plus make-up water) enters the steam generator through an inlet nozzle which is disposed in the upper portion of the cylindrical shell. The feedwater is distributed and mixed with water removed by the moisture separation and then flows down an annular channel surrounding the tube bundle. The feedwater then reverses direction and passes up around the outside of the tubes of the tube bundle where it absorbs heat from the reactor coolant flowing within the tubes. The heat absorbed causes the feedwater to boil and produce steam. The steam produced by the boiling water rises into the steam drum section. The moisture separator then removes the water entrained within the steam before it exits from the steam generator through a steam outlet. The steam then flows to the steam turbine which is connected to an electrical generator. Subsequently, the steam from the steam turbine is condensed and re-routed back into the steam generator to continue the flow cycle.
The U-shaped tubes are supported at their open ends by conventional means whereby the ends of the tubes are seal welded to a tube sheet which is disposed transverse to the longitudinal axis of the steam generator. A series of tube supports arranged in spaced relationship to each other are provided along the straight portion of the tubes in order to support such portion of the tubes. An upper tube support assembly is utilized to support the U-shaped portion of the tubes of the tube bundle. The upper assembly comprises a plurality of retainer rings arranged around the outside of the tube bundle in spaced relationship to each other.
The retaining rings, like the tube supports, are arranged substantially transverse to the longitudinal axis of the steam generator. Each retaining ring is generally of an oval shape which coincides with the outer periphery of the tubes of the bundle at the particular location of the retaining ring. Thus, the size of the oval of the retaining rings decreases with the distance toward the upper end of the tube bundle. The uppermost retaining ring, therefore, is relatively small inasmuch as it is located at the uppermost portion of the tube bundle where the shape of the tube bundle is rapidly converging.
Each of the retaining rings is connected to a plurality of antivibration bars which are typically disposed between each column of the U-shaped portion of the tubes. In some steam generators, the antivibration bars comprise a bar bent into a V-shaped configuration such that two legs are formed with an angle therebetween. The V-shaped bars are inserted between successive columns of the steam generator flow tubes. The V ends of the bars are inserted between the flow tubes; the free ends of the bars are welded to opposite sides of the appropriate retainer ring. In this manner, each of the tubes of the tube bundle is supported along the length of the curved or U-shaped portion at a number of spaced locations by an antivibration bar. This arrangement provides line support and yet allows the feedwater to flow around and between the curved portion of the steam generator tubes. In other words, the antivibration bars provide support and do not substantially interfere with the flow of the feedwater.
The antivibration bars are intended to prevent excessive vibrations of the individual tubes of the entire tube bundle. It is well known that the vibrations in question are caused by flow of the water and steam past the flow tubes. These flow-induced vibrations can potentially damage the flow tubes. It is also well known that the U-shaped portion of the tube bundle is severely affected by the vibrations. And, because of the bent configuration, the U-shaped portion of the tube bundle is the most difficult to adequately support in order to eliminate the flow-induced vibrations. Further, it is well accepted that current hydraulic and dynamic response technology can neither exactly define nor completely eliminate the root cause of the vibrations. It has been, therefore, left to mechanical means to attempt to completely or at least substantially eliminate the vibration issue. While the advent of the antivibration bars or similar technology has materially reduced the magnitude and presence of vibration, they have not in the prior art completely eliminated damaging vibrations.
The mechanical aspects of the curved or bent portion of the tubes of the tube bundle are the major obstacles in the way of a mechanical solution to the problem.
The U-shaped tubes of the tube bundle have dimensional tolerances associated with their outer diameter. There are also dimensional variations caused by ovalization of the tubes as a result of the bending. Furthermore, the spatial relationship between adjacent tubes is a variable, albeit within set design limits. Thus, there is a dimensional tolerance associated with the nominal spacing between the steam generator tubes. There is also a dimensional tolerance associated with the outer dimensions of the prior art vibration bars, which as explained above, typically comprise rectangular bars. They may also comprise a square, an oval, or any other shape having a uniform or a nonuniform cross-sectional shape. However, notwithstanding the particular shape chosen, there is the dimensional tolerance associated with the size of the bars. The combination of these tolerances and dimensional variations prevents the elimination of gaps between the antivibration bars and the tubes of the steam generator. Any gaps are, of course, undesirable because they allow vibration of the tubes and relative motion between the tubes and the antivibration bars. The relative motion can cause wear and subsequent failure to the tubes of the steam generator. There have been numerous attempts in the prior art to minimize gaps. Unfortunately, decreasing the size of the gaps only decreases the magnitude of this issue, it does not eliminate the issue.
In one prior art application, a method was disclosed whereby hollow antivibration bars are expanded in place between the rows of steam generator tubes to eliminate the gaps due to dimensional variations. Some obvious limitations of this method include difficulty of using the method with previously operated steam generators which may be or are radioactive, and the difficulty in general of controlling the expansion in order to obtain acceptable gaps. Also, the hydraulically expandable bars must have a relatively thin wall for the expansion to occur to close the gaps, the thin wall is of concern if wear does occur.
Another prior art attempt to eliminate the gaps utilized compliant antivibration bars whereby bars which are slotted in the plane of columns of tubes present a compliant support for the tubes in contact with the bars. The slotted bars are then inserted between columns of tubes and attached at their ends to the retainer rings surrounding the exterior periphery of the bundle of tubes. While this unique apparatus is a further step in the right direction toward minimizing or eliminating gaps between antivibration bars, it too has its limitation. The need to make the bars compliant requires a relatively thin wall which is of concern if wear does occur. Also, the bars may be required to flex in alternating directions relative to its nominal centerline in order to accommodate the various sizes of the steam generator tubes which also requires a relatively thin wall.
A further prior art attempt to minimize the gaps between the antivibration bars and the steam generator tubes comprise expandable antivibration bars. In this prior art attempt an expandable support was disclosed whereby the antivibration bars are split along the plane of the columns of tubes and are provided with one or more sets of mating inclined planes at the split surfaces. The split halves allow relative motion between the halves of the antivibration bars while the inclined planes provide a means to increase or decrease the overall thickness of the antivibration bars. In this manner it was hoped that the space between columns of steam generator tubes could be fitted with antivibration bars whose thicknesses coincide with the actual distance between the rows of tubes. However, because the steam generator tube diameters vary significantly throughout the columns of tubes, the two parallel plane surfaces of the expandable bars may result in locations with gaps larger than desired.
Accordingly, there still exists a need for other and better methods to minimize or eliminate the gaps between the steam generator tubes and the antivibration bars in order to prevent damaging vibrations of the steam generator tubes and to eliminate any relevant motion between the antivibration bars and the steam generator tubes.
Accordingly, an object of the present invention is to provide a method and apparatus which prevents operational vibrations of the tubes of the steam generator.
Another object of the present invention is to provide a method of manufacturing antivibration bar apparatus which virtually eliminates any gaps between the antivibration bar and the tubes of a steam generator.
Additional objects and advantages as well as the advancement of the art are illustrated and disclosed in the present invention over the prior art and will become apparent in accordance with the following Summary of the Invention which is not intended to limit the scope of the invention which is expressed in the claims which follow the description herein.